1. Field of the Invention
This patent relates to a method for attaching a plastic part to a composite container using induction heating. More particularly, this patent relates to a method for induction sealing a plastic part to a composite container by positioning the plastic part onto the top portion of the container and exposing the top portion of the container to electromagnetic fields generated by a pair of linear shaped coils.
2. Description of the Related Art
Composite containers are commonly used to hold drink mixes, snacks and other foods. A typical composite container has a cylindrical body, a plastic or metal bottom and a plastic overcap. The container body is comprised of a polyfoil inner liner, a paperboard structural layer and a paper outer label. The polyfoil inner layer has a moisture-proof thermoplastic layer that contacts the container contents, a metallic foil layer adjacent the thermoplastic layer, and an outer paper layer. A plastic seal may be positioned under the plastic cap and sealed to the top edge of the container to help maintain the freshness of the contents and extend its shelf life. Alternatively, the plastic cap may be sealed directly to the container body.
Sealing a plastic seal or cap to a composite container body is often accomplished by heating the plastic seal or cap while pressing it onto the container body. Various means of heating the plastic seal or cap are known, including radiant heating and conductive heating, but the most successful means, at least with respect to composite containers, is induction heating.
Induction heating is a process that relies on electrical currents within a material to produce heat. Induction heating requires a source of electrical energy, an induction coil and an electrically conductive target material (the material to be heated). As electricity passes through the induction coil, an electromagnetic field is generated around the coil. The shape of the electromagnetic field depends on the shape of the coil. When the target material is exposed to the electromagnetic field, eddy currents are created in the target material, causing the target material to heat up because of resistance losses. The heat from the target material is transferred to adjacent plastic materials, causing them to soften and fuse together. The plastic materials form a seal upon cooling.
When sealing a plastic cap onto a composite container, it is desirable to obtain a broad seal to prevent breaching of the seal. A broad seal is defined as a seal at least xc2xc inch wide, and preferably at least xc2xd inch wide. It is also desirable to be able to seal containers at high production speeds. High production speed is defined as at least 100 containers per minute. Unfortunately, most commercially available induction sealing machines provide extremely localized heating. As a consequence, it is difficult to obtain a broad seal between a plastic part and a composite container at high production speeds.
Thus, it is an object of the present invention to provide a method and apparatus for induction sealing a plastic part to a composite container.
Another object is to provide a process for forming a broad seal between a plastic part and a composite container at faster speeds than conventional induction sealing processes.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.
The present invention is a method and apparatus for induction sealing a plastic part to a composite container. The method includes the steps of:
providing a composite container having a body, the body having a top portion terminating in a top edge, the body defining a cylinder extending beyond the top edge, the body comprising an outer structural layer, an intermediate layer of electrically conductive material and an innermost layer of thermoplastic adjacent the electrically conductive layer;
positioning a plastic part onto the composite container such that it forms a friction fit with the inside of the top portion of the container body;
exposing the top portion of the container body to a pair of electromagnetic fields created by a pair of induction coils arranged in parallel on either side of the container, each coil doubling back on itself so as to form two substantially parallel linear sections, one linear section of each coil being positioned above the container top edge and intersecting the cylinder defined by the container body, the other linear section being positioned below the top edge and lateral to the container body, whereby said exposure induces an electrical current in the electrically conductive layer of the container body, thereby generating heat that raises the temperature of the adjacent thermoplastic layer to a temperature suitable for adhering the thermoplastic layer to the plastic part;
rotating the container while it is being exposed to the electromagnetic fields; and
forming a seal between the plastic part and the thermoplastic inner layer of the container by removing the top portion of the container from the electromagnetic fields and allowing the thermoplastic layer to cool and solidify.
The parallel linear sections of each coil create overlapping electromagnetic fields. As the container is exposed to the overlapping electromagnetic fields, the container is carried on a moving conveyor belt and rotated to assure even heating of the thermoplastic liner and plastic part.